Using Wideband CDMA (WCDMA) standards and nomenclature as an example, a “Radio Network Controller” (RNC) receives uplink traffic from users supported by one or more “NodeBs” that are associated with the RNC. Each NodeB transmits downlink signals to and receives uplink signals from each user and the RNC in turn provides connectivity to the supporting Core Network (CN). In this context the term “user” denotes a cellular telephone or other item of user equipment.
Uplink enhancements specified in Release 6 and later 3GGP standards for WCDMA aim for performance improvements on the uplink, based primarily on deciding which users transmit on the uplink at which times. This uplink scheduling—referred to as “Enhanced UpLink” or “EUL”—provides increased system capacity and throughput through reductions in uplink interference, particularly among high-rate users.
NodeBs “schedule” user transmissions on the EUL using a scheduling “grant table” that includes a number of pre-defined numeric values, referred to as “table entries.” Table entries are numeric values representing the maximum power that can be used by any given user, for transmitting on the Enhanced-Dedicated Physical Data Channel(s) or E-DPDCHs. Each NodeB schedules users for uplink transmissions by making absolute Grants from the grant table, or by making so called relative grants, which are relative to absolute grants. A user receiving an uplink grant uses the grant value to compute its “serving grant” based on the maximum power indicated by the grant, and other information provided to the user at connection setup. While the table entries are not literal data rate values, each entry maps to some maximum data rate for any given user, depending on a number of variables like the transport channel format(s) in use, the Transmit Time Interval (TTI) duration, etc.
In some instances, the air interface carrying the users' uplink transmissions is not the bottleneck as regards the overall throughput of uplink traffic for a given NodeB. Instead, the “transport network” on the backhaul side of the NodeB can become the point of congestion. Particularly, the backhaul links connecting the RNC to each NodeB may have an aggregate data throughput that is less than the best-case EUL throughput supported over the air interface by the NodeB. For example, connecting an RNC and a Node-B with a 2 Mbps link leaves that link susceptible to uplink congestion because the aggregate data rate supported by the EUL can easily exceed 2 Mbps. A network operator might nonetheless find good reasons to use a 2 Mbps link on the backhaul, such as cost, convenience, etc.
The known problem of transport network (TN) congestion in such cases has led to the advent of TN flow control, wherein the RNC monitors TN congestion levels (related to EUL traffic) and sends corresponding congestion indicators to the affected NodeBs. As one example, see Szilveszter Nádas, Zoltán Nagy and Sándor Rácz, “HSUPA transport network congestion control,” (Presented at 4th IEEE Broadband wireless access workshop co-located with Globecom 2008), ETH-08:000588 Uen. The main goal of the EUL TN flow-control is to somehow adapt bit rates of the uplink data flows contributing to the congestion, as needed to relieve the congestion. Correspondingly, any given NodeB responds to congestion indicators by reducing the uplink data rates (bit rates) of at least some of the users it is supporting. To do so, the Node-B informs given users about their maximum allowed bit rates using absolute and relative grants.
The EUL scheduler in the NodeB assigns an absolute grant to the UE, and that grant is valid until a new absolute grant arrives. The user is allowed to send data with this granted rate—the absolute grant is expressed as an E-DPDCH/DPCCH power ratio, so it indirectly determines the user's maximum allowed uplink sending rate. Problematically, however, the grant table contains a limited number of possible grant rates. Hence, the scheduler's ability to “tune” its grant adjustments responsive to TN congestion indicators is limited.